1. Field of the Invention
This invention relates to a method for applying solder to flexible circuits based on a hot air solder levelling approach. More specifically, it relates to using a negative pressure region at the hot air nozzle exit to reduce the distance between the panel and the hot air nozzle for obtaining interaction of the panel with air current patterns.
2. Description of Related Art
In a standard manufacturing process for printed circuit boards (PCBs), copper conductive tracks on the PCB are protected from oxidation by coating them with a thin layer of solder. The tracks are first cleaned in a flux solution which comprises a mixture of acid cleaner and solder solvent, and then dipped into a bath of molten solder. On withdrawal from the bath, solder adheres to the exposed copper tracks (but not to other regions of the PCB). To ensure that an even coating of solder has been applied, a hot air solder levelling (HASL) machine, such as that described in GB 1457325, can be used. In a HASL machine, as the PCB is lifted out of the solder bath, it is blasted by jets of hot air from nearby nozzles which blow away excess solder.
After the HASL stage, electronic components such as silicon chips are mounted onto the PCB. Typically, a set of pads is arranged around the periphery of a component site, each pad being the termination of a conductive copper track that provides an input/output line to the component. The components usually have a set of leads for soldering to the pads, or occasionally terminals on the component can be directly bonded onto the pads (such as with direct chip attachment). In either approach, additional solder is applied to the pads after the HASL process, typically by screening. This solder is used for bonding the components to the pads. As the size of the electronic components for such circuits has shrunk, so has the spacing of the pads surrounding them. In some cases adjacent pads are no more than 0.2 mm apart, approximately equal to the width of the pads themselves. Solder screening techniques are not at present capable of depositing such finely-spaced quantities of solder.
In order to produce flex circuits with a very fine pad spacing, an attempt was made to use the HASL process not only to prevent oxidation of the copper tracks, but also to provide sufficient solder on the pads to allow the components to be attached without a further solder apply stage. This places constraints on the thickness of the HASL solder coating, because too much solder will result in shorts between adjacent pads, while too little will not provide a good bond. The specifications for the particular flex circuit in question were a solder pad height of between 10 and 70 .mu.m (microns) (height is used as an accessible measure of the volume of applied solder). However, a significant proportion of the panels required rework (because it is much easier to remove excess solder at rework than to add some more, the process was deliberately set to ensure that most out-of-spec pads had too much solder). The high degree of rework required costs both time and money.